I cannot see or render an image as the world background in cycles
To use a flat image, (really not recommended, you don't gain any dynamic range or depth of the environment) you will need to use an image texture node rather than an environment texture node and set the texture coordinate node to window so that the image fits within the camera bounds at all times.
There is a significant drawback to this, in that using an image this way does not provide the lighting that an HDRI does provide, and therefore it is better to use an actual HDRI that contains the lighting information and that captured the correct equirectangular projection that the environment image should have. See the gifs below for an example of the differences.

Regular photo, it contains no real light information, uses window coordinates stays static while the camera rotates:

HDRI with light information, correct projection as would be expected of the world when camera is moved:


Either of those two options will solve your current dilemma, if you do not have any HDRI images, there are plenty of resources for free ones, see our Resources for Blender post here for some good ones.